backend polish: stats LEFT JOIN, CSV formula injection, trustProxy, validated env vars, dead code

- Stats: bars table is now the LEFT side of the join to transactions,
  so a bar with zero sales still gets a zero row instead of vanishing
  from the totals table until its first sale (indistinguishable from
  a deleted bar). by_day stays JS-computed on purpose — a SQL rewrite
  would trade DST-aware timezone handling for a fixed-hour-offset
  'localtime' expression that's wrong on DST transition nights, to
  fix a cost the original review noted is 'fine today'. Not worth
  that trade for a money-adjacent report; left a comment explaining
  why.
- CSV export: cells starting with =/+/-/@ are now prefixed with '
  before quoting, closing a formula-injection path (an admin-entered
  drink/bar name like =HYPERLINK(...) would otherwise execute when
  the export is opened in Excel/LibreOffice).
- server/index.ts: PORT is now parsed and range-checked instead of a
  bare Number(...) (an unparseable value silently became NaN, and
  Fastify listens on a random free port for that); ADMIN_PASSWORD
  missing now warns at boot instead of only surfacing as a 500 at
  the first login attempt; new WUTZ_TRUST_PROXY env flag (off by
  default) so req.ip can actually reflect the real client behind a
  reverse proxy, documented in the README alongside the other env
  vars.
- time.ts: WUTZ_DAY_CUTOFF_HOUR gets the same parse+range-check
  treatment, for the same reason (a typo used to silently disable
  the business-day rollback with no error).
- shared/src/index.ts: Drink.archived is now typed 0 | 1, matching
  what SQLite actually returns (was boolean, which only worked by
  accident since 0 is falsy); removed TransactionRecord/
  TransactionItemRecord, declared but never returned by any route —
  leftovers from a planned endpoint that was never built.

Verified: pnpm --filter server|client typecheck/build all clean;
also ran the built server with a bad PORT and no ADMIN_PASSWORD to
confirm both warnings fire and the port falls back correctly.

README.md

@@ -164,6 +164,9 @@ the service user owns the database directory automatically.
 - `ADMIN_PASSWORD` (**required** for backoffice login)
 - `WUTZ_TZ` (default `Europe/Berlin`) — timezone for stats display and grouping
 - `WUTZ_DAY_CUTOFF_HOUR` (default `5`) — sales before this local hour count toward the previous business day
+- `WUTZ_TRUST_PROXY` (default off, set to `1` to enable) — trust `X-Forwarded-*` headers for
+  `req.ip`. Only turn this on if you're actually running behind a reverse proxy (see the
+  systemd/deploy section) — otherwise a client can spoof its own logged IP.
 - `WUTZ_SERVER_PORT` (default `3000`, **client dev only** — not read by the server) — the port
   `dev:client`'s Vite proxy targets; set it to match `dev:server`'s `PORT` when running the server
   on something other than the default